This invention generally relates to the field of antennas, more particularly, antennas that are used in small communication devices.
The growth of commercial radio communications and, in particular, the explosive growth of cellular radiotelephone systems has resulted in extensive use and handling of mobile phones by subscribers. One of the important considerations in designing a small communication device, such as a cellular phone, is the physical characteristics of its antenna. Typically, it is desirable to design a small antenna that is flexible enough to withstand day-to-day handling, including occasional mishandling. For example, the antenna should tolerate significant bending stresses that could bend it up to 180.degree. and still return to its original shape when the bending stresses are removed.
Conventional antennas use a radiating element that is overmolded with a resilient material, such as plastic or elastomer, to make it flexible. The radiating element may be comprised of wire, stamped, or etched metal. Etched flexible circuits are also used as the radiating element. Conventional overmolding techniques with plastic or elastomer, however, produce an antenna structure that is difficult to match to the bending and elongation characteristics of the metallic radiating element. Thus, bending the antenna, especially at low or high temperature, produces excessive shear stresses at the interface of the radiating element and the overmolded structure. As a result, current antenna designs often provide limited flexural endurance lifetimes. As a compromise, larger metallic elements and/or overmolded structures are used, with a resulting sacrifice in the size of the antenna. Also, some conventional antennas use relatively rigid metallic sheets, for example, metals in solid sheets, that are placed in various positions on the antenna assembly to produce the antenna's electrical structures, such as ground planes, tuning elements, etc. However, the use of rigid metallic sheets substantially reduces antenna flexibility.
Moreover, some mobile communication devices use retractable antennas. A retractable antenna must be rigid enough to allow for insertion of the antenna into a clearance area without buckling. Conventional antennas employ a circular wire or rod as their primary structure. This rod may serve as a radiating element or merely as a support for the radiating element. Typically, the rod gets inserted into a discrete tube or guiding feature disposed within the housing of the device. Rod shaped antennas, however, require a large clearance area, which reduces the available space for other radio circuitry.
Therefore, there exists a need for a rigid and thin antenna that has superior flexibility.